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What is alchemy?

The cheesemakers were probably none too impressed, but the rest of the world should be eternally grateful. It was 17 February 1869 and Russian chemist Dmitri Mendeleev was due to do some consultancy work at a cheese factory in St Petersburg. But he cancelled and spent the day scribbling feverishly at home. By the evening he had the outline of one of the most successful scientific theories of all time: the periodic table of the elements.

Mendeleev’s eureka moment was the culmination of centuries of work trying to understand and control the processes of material change. What happens when a candle burns? Why does a pinch of salt disappear when stirred into a glass of water? Can lead be turned into gold? We now recognise those questions as belonging to the realm of chemistry, which has a reputation as a rather dull and sober science. But its origin was anything but.

The first steps were made by philosophers in ancient Greece. Aristotle asserted that everything was made from four elements: earth, fire, air and water. Materials had particular qualities because of the proportion of these elements that they contained. A metal, for instance, was made from earth and water, but if you heated it some of the earth changed to fire.

Aristotle died in 322 BC, a decade after Alexander the Great conquered Egypt and established a new capital, Alexandria. Artisans steeped in Aristotelian philosophy began to dabble in metallurgy and dye-making. They called their craft khymeia, meaning “cast together”. The tradition was later passed to Islamic scholars, who called it al-khimya. Their knowledge eventually found its way to medieval Europe, where practitioners of magick wrapped it in mysticism and called it alchemy or just chymie.

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The alchemists’ main goal was the philosopher’s stone, a substance which could transmute base metals into gold and silver, cure any disease and held the key to eternal life. They were also craftsmen who used their expertise at manipulating and transforming materials to produce medicines, glass and explosives.

But alchemy wasn’t a science. That turning point came in 1661 when philosopher Robert Boyle published a groundbreaking book called The Sceptical Chymist, which applied the newly-minted scientific methods to chymie. Boyle argued that you couldn’t just assert that matter was made of four elements; you had to prove it with repeatable experiments.

Taming the elements

The man who provided them was French aristocrat Antoine Lavoisier. He took on Boyle’s challenge and went searching for elements, which he defined as anything that could not be broken down further. In 1789 Lavoisier published a list of 33 “elements”, many of which actually are elements as we understand the concept today. Many more were soon discovered. The idea that each element had its own unique atom became popular, as did the idea that the elements combined with each other to form compounds.

By Mendeleev’s time, 63 elements were known. His breakthrough was to organise them into groups by atomic weight and thereby reveal some patterns in their properties. Group 1, for example, were all soft metals that react violently with water. Group 7 included the gases fluorine, chlorine and bromine, which exist as molecules composed of two atoms. That was not the only pattern. Within each group the reactivity of the elements changed as the atoms got heavier. In group 1, reactivity increases as the atoms get heavier. But in group 7 the opposite is true.

The periodic table is the unifying theory of chemistry. It didn’t just explain observations, it also made predictions. Where there was no element with the correct properties, Mendeleev boldly left a gap, claiming that a new element would be discovered to fill it. He was right. For example, there was a space immediately below silicon. Mendeleev called it “eka-silicon” and 15 years later German chemist Clemens Winkler discovered it. He called it germanium.

The electron

It was not long until the underlying cause of the patterns was discovered: the electron. The particle was discovered in 1896 but the crucial experiments were done by Hans Geiger and Ernest Marsden a decade later. They fired a stream of helium nuclei at a piece of gold foil. Unexpectedly, many of the nuclei passed straight through, leading to the conclusion that gold atoms were mostly empty space. Their interpretation, which later proved essentially correct, was that the electrons were orbiting the nucleus, leaving huge tracts of nothingness in between.

The orbits of the electrons explain an element’s chemical and physical properties. Reactivity, for example, depends on how easily an atom can gain or lose an electron.

But the electron is something of a double-edged sword. For all the regularity it introduced, the shorthand chemists use to understand its effects is just an approximation. In truth, electrons are quantum objects with weird properties: they can be in two places at once, or “tunnel” through space.

As the quantum revolution gathered pace, scientists also started to probe the atomic nucleus in detail. One of their key discoveries was that elements could be “transmuted” from one to another by nuclear reactions – something that appeared forbidden by the laws of chemistry. Nobody mentioned the A word, but in 1951 chemist Glenn Seaborg took a base metal, bismuth, and turned it into gold.